Apparatus used for MRI diagnostic procedures require the use of large magnets to create the strong, uniform magnetic field required for accurate test results. An MRI unit must be of a size and configuration to enable a patient to place all or a substantial part of his or her body into that portion of the magnetic field found to be the strongest and most uniform.
A typical MRI apparatus features an opposed pair of magnet groupings supported by a generally C-shaped frame designed to hold the groupings in face-to-face parallel configuration. An air gap or space is defined between the magnet groupings where the patient is positioned. Each of the frame legs and cross members is made from a highly permeable ferromagnetic material to complete a circuit for the magnetic flux to travel from one magnet grouping across the air gap and returning to the remaining grouping.
Because the magnet groupings are quite heavy, it is necessary to construct the frame of sufficiently massive members to limit deflection of the frame not only by the weight of the magnets but the weight combined with the attractive force the magnet groupings exert on each other. The weight of the magnet groupings and the massiveness of the frames needed to support them can make MRI units costly and difficult to construct and inconvenient to move.
MRI units and the circular magnet groupings used in them are well represented in the prior art.
U.S. Pat. No. 5,623,241 (Minkoff) teaches and describes a permanent magnetic structure comprising a C-shaped frame which supports two opposed magnet groupings at the open end of the frame legs, thereby forming an air gap. The groupings in Minkoff are single-piece, permanent magnets arranged in parallel, face-to-face relationship, each magnet having a pole piece positioned on its outer face.
U.S. Pat. Nos. 4,943,774 and 5,134,374 (Breneman, et al.) teach and describe various magnetic field control apparatus. Both patents feature magnet groupings consisting of a permanent magnetic pole supported on a rear frame and having a pole face formed from ferrous material. Breneman, et al '774 utilizes a number of ferrous segments positioned on an annular member attached to one pole face, with the segments being radially repositionable to adjust the magnetic field strength and orientation. Breneman, et al '374 uses a series of segments applied directly to the pole faces to adjust the magnetic field.
U.S. Pat. No. 5,194,810 (Breneman, et al.) teaches and describes a superconducting MRI magnet with magnetic flux field homogeneity control which uses radially positioned circular segments as shims to adjust the magnetic field created between two magnet groupings.
U.S. Pat. No. 5,332,971 (Aubert) teaches and describes a permanent magnet for nuclear magnetic resonance imaging equipment utilizing a number of magnetic blocks arranged into concentric rings to produce a homogeneous magnetic field.
U.S. Pat. No. 5,659,250 (Domigan, et al.) teaches and describes a full brick construction of magnet assembly having a central bore in which a plurality of individually magnetized bricks are arranged in elliptically shaped sections to create a bore within which a patient can be placed for MRI examination. The individual bricks are formed as parallelepiped and the field is adjusted by the shapes of the groupings of bricks used.
U.S. Pat. No. 4,998,084 (Alff) teaches and describes a multipolar magnetic ring consisting of two rings having magnetic segments formed about their inner peripheries which interengage to form a single ring.
U.S. Pat. No. 4,734,253 (Sato, et al.) teaches and describes a method for the preparation of sintered magnets from Fe-B rare earth alloy with the sintered magnetic segments arranged to form a circular ring.
U.S. Pat. No. 4,538,130 (Gluckenstern, et al.) teaches and describes a tunable segmented ring magnet and method of manufacture whereby a circular ring of permanent magnetic segments is formed for use in NMR imaging systems.
U.S. Pat. No. 4,093,912 (Double, et al.) teaches and describes an NMR magnet assembly with pole face parallelism adjustment used to align the faces of magnetic pole pieces used in NMR magnet arrangements.
U.S. Pat. No. 5,028,903 (Aubert) teaches and describes a spherical permanent magnet with equatorial access consisting of a pair of hemispherical magnet arrangements creating therewithin an air gap for use in MRI procedures.
It is an object of the present invention to provide a permanent magnet arrangement for use in MRI equipment in which the strength of the magnetic field created by the magnets used in the arrangement is increased without significantly increasing the weight of the arrangement.
It is a further object of the present invention to provide a field-strengthening arrangement adaptable to a wide variety of magnet segment arrangements.
Yet another object of the present invention is to provide embodiments of the present invention that are inexpensive to construct and maintain.
An additional object of the present invention is to provide such magnetic field enhancements which take full advantage of the adjustability of concentric ring magnet arrangements.